The present invention relates to vehicle wheel alignment, and more particularly to vehicle wheel alignment systems which measure the locations and orientations of the vehicle wheels in a three dimensional coordinate system.
European Patent Application PCT/US93/08333, filed in the name of Jackson and published under the Patent Cooperation Treaty as WO 94/05969 (hereinafter referred to as 94/05969) has issued in the U.S. as U.S. Pat. No. 5,535,522. U.S. patent application Ser. No. 08/651,766, filed May 22, 1996, now U.S. Pat. No. 5,675,515 discloses in detail certain improvements to the invention described in the 94/05969 application. The full disclosures of all three of these documents are incorporated herein by reference.
U.S. Pat. No. 5,535,522 discloses that measurements are made of the images formed in video cameras of optical targets which are mounted to a wheel of a vehicle. As shown in FIG. 3 of that patent and described therein, the target is a flat plate on which appears an array of circles. See, for example, column 9, lines 27-37:
"An example of a typical target 54 that can be used on the wheels of the vehicle 20 is illustrated in FIG. 3. As can be seen from this figure, the target consists of a flat plate with a pattern of two differently sized circles 62, 63 marked in a pre-determined format thereon. Although a specific pattern is shown in this figure, it will be evident that a large number of different patterns can be used on the target 54. For example, the target need not be circular, a larger or smaller number of dots may by included. Moreover, other sizes and shapes can be used for the dots. In addition, multifaceted plates or objects can also be used for the targets."
Other than the last sentence cited above, U.S. Pat. No. 5,535,522 does not disclose or discuss any other shape for the elements of the targets (i.e. the "dots") than circles, nor does it disclose or discuss any advantages or disadvantages of using any shape of target elements other than circles.
To determine the position and orientation of the target, measurements must be made of the visual image of the elements of the target. See, for example, column 9, lines 50-60 of the '522 patent:
"The apparatus could, for example, calculate the center of each of the circles 62a, 62b by means of, say, a method called centroiding. This is a method commonly used by image analysis computers to determine the positioning of the center point or center line of an object. Once the center points of the two circles 62a, 62b have been determined, the distance between the two can be measured. This process is then repeated for other circles in the pattern on the target 54. These distances can then be compared to the true distances (i.e. non-perspective distances) between the respective centers. Similarly, the angle to the horizontal (or vertical) of the line joining the two centers can be determined. Once allowance has been made for the effect of the focal length of the lens 40 and other optical characteristics of the components such as beam splitter 42, mirror 44 and mirrors 46L, 46R are considered, a calculation can be made as to what the orientation of the target 54 is."
As described in U.S. Pat. No. 5,535,522 and as is well known in the art, the circles of the target project onto the detected image as ellipses. Measuring parameters in a video image of an ellipse, such as location of centroid, lengths of major and minor axes, and orientation of major and minor axes, cannot be performed with resolution and accuracy sufficient to provide acceptable wheel alignment measurements.
What is needed is video alignment system including target elements which allow much higher precision in the measurement of the detected image, thereby providing much higher resolution and accuracy in the resulting wheel alignment measurements. Further, what is needed is a scheme for determining the quality of the measurements of the detected image, such that the measurements can be refined and improved resolution and accuracy results. Still further, what is needed is an improved ability to determine, in a video image, which detected objects constitute images of the elements of the optical target. It would also be desirable to have an improved scheme for using optical targets to locate the surface on which the vehicle wheels roll.